I get this in my logs all the time and the more I get it, the slower my system gets. It literally brings the whole system to its knees to where I can't even move the mouse. I did some research on the net and found other people with similar problems. It seems to have to do with using an integrated graphics chipset and it may be limited to AMD APUs. I added a second monitor to my system and this caused the problem to become pronounced enough that the system was unusable. I have 8GB of RAM (if that matters), and the problem doesn't seem to occur with the Ubuntu's 3.13 kernel and so I've reverted back to it.
This is not specific to radeon. See this thread: http://lists.freedesktop.org/archives/dri-devel/2014-August/065842.html You can disable CONFIG_DMA_CMA in the meantime.